Re: [musl] Triggering Overflow (or Underflow) without triggering Inexact on i386

[Szabolcs Nagy <nsz@port70.net>]

* Damian McGuckin <damianm@esi.com.au> [2020-01-21 08:11:50 +1100]:
> I am trying to write arithmetic expressions using only x87 instructions to
> trigger an exception.  The ASM instruction to raise the flag in the status
> register has way too much overhead.
> 
> My normal technique of doing either of these would also trigger Inexact.
> 
> e.g.
> 	volatile long double t;
> 
> 	Overflow:  t = largest normal long double, t *= t;
> 	Underflow:  t = smallest normal long double, t *= t;
> 
> Can it be triggered WITHOUT also triggering INEXACT?

ieee operations raise inexact and underflow/overflow together.

so you cannot implement feraiseexcept by only doing arithmetic
ops (which can do individual underflow/overflow), but that's a
rare function so it can be slow fldenv/fwait etc.

note that there is __math_uflow/__math_oflow now for internal
math code (and they don't use the smallest/largest normal,
instead they use small/large numbers with simple representation
since sometimes those can be constructed as immediates on some
targets, but this is a micro-optimization).